Australia's consumer watchdog will fine Optus $100 million after the telecommunications company was found to have engaged in unconscionable conduct by selling products that consumers did not want or need.

The Australian Competition and Consumer Commission (ACCC) today said the fine, which is subject to Federal Court approval, comes after Optus admitted that its sales staff acted unconscionably when selling phones and contracts to over 400 consumers at 16 different stores across Australia between August 2019 and July 2023.

Many of the affected consumers were vulnerable or experienced disadvantage, such as living with a mental disability or diminished cognitive capacity.

Some customers were financially dependent or unemployed, had limited financial literacy or English was not their first language.

Others were First Nations Australians from regional, remote and very remote parts of the country.

According to the ACCC, sales staff pressured customers into buying phones and accessories they did not need and could not afford.

The ACCC said concerns were raised that customers did not understand their ongoing payment obligations, while some were led to believe the goods were free or included as a bundle at no additional cost.

It was alleged in some cases sales staff had no regard for whether customers had Optus coverage where they lived or whether they could afford the products.

Optus has signed an undertaking, accepted by the ACCC, to compensate impacted customers and improve its internal systems.

Optus CEO Stephen Rue said the misconduct was inexcusable and unacceptable.

"I would like to sincerely apologise to all customers affected by the misconduct in some of our stores," Rue said.

"Optus failed these customers, and the company should have acted more quickly when the misconduct was first reported.

"I am leading the implementation of extensive changes across the company with active responses to the issues raised well under way.

"However, there is much more to do as we work to regain our customers' trust and improve support and protections for them, especially for those who are vulnerable."

Rue said the company has made numerous executive and senior leadership changes.

The company has also undertaken disciplinary action in some cases, terminating retail sales staff who were found to be responsible for inappropriate sales practices.

"This is not what Optus stands for and we will hold ourselves to a higher standard going forward," Rue said.

In other changes to the business, the business will roll out additional training for frontline staff, more robust credit check processes and a new network coverage check tool to better inform consumers about making purchases.

Optus has also agreed to pay $1 million to support digital literacy initiatives for First Nations Australians.

DOWNLOAD THE 9NEWS APP: Stay across all the latest in breaking news, sport, politics and the weather via our news app and get notifications sent straight to your smartphone. Available on the Apple App Store and Google Play.